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Is the equation Pressure = P(atmosphere) + (density(water).hight.gravity) correct? Or does that apply only for the pressure aplied on the bottom of the system?

(I'm sorry about the bad english)

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- Thread starter MartaBraga
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- #1

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Is the equation Pressure = P(atmosphere) + (density(water).hight.gravity) correct? Or does that apply only for the pressure aplied on the bottom of the system?

(I'm sorry about the bad english)

- #2

Doc Al

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That equation is correct. Realize that "height" is really *depth *below the water surface.

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- #4

mgb_phys

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(the force sideways is the same as the force downwards because it is a fluid, so pushes in all directions)

Cyrus's reply is about calculating the TOTAL force on the wall.

At the bottom is the pressure you calculated, at the surface there is obviously no pressure and half way down is half the pressure.

- #5

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How do I do that, I mean, calculate it through the area of the triangle? (I rather do anything than integrate!)

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- #7

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I calculated the pressure through that formula and then divided it by 2 and multiplied it by the area... and the result is correct... But I thought I had to use the atmospheric pressure in the formula...

Or is this pure luck?

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- #9

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I calculated the pressure = 1000 (the density) x 40 (the height) x 9,8 (the aceleration of gravity. The result is 392000, which I divided by 2 = 196000. I multiplied this value by the area of the dem in contact with the water, 6000m2. The result is 1,176x10^9.

This was the result I was looking for.

- #10

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Yes that exactly right. Do you know what you are doing though?

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(I'm really sorry about the grammar errors, but I'm Portuguese, so there are some words and expressions I don't fully know how to spell or use)

- #12

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You can leave out the atmospheric pressure because it is applied to the downstream side of the dam also - hence it cancels out. If there were no water in the lake at all, just the air on both sides, there would be no net force on the dam.

- #13

Doc Al

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- #14

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(I'm really sorry about the grammar errors, but I'm Portuguese, so there are some words and expressions I don't fully know how to spell or use)

Hint: The area of a triangle is 1/2*(base)*(height)!

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